There is provided a process for producing alkylaromatics, especially ethylbenzene and cumene, wherein a feedstock is first fed to a transalkylation zone and the entire effluent from the transalkylation zone is then cascaded directly into an alkylation zone along with an olefin alkylating agent, especially ethylene or propylene.
Ethylbenzene is a valuable commodity chemical which is currently used on a large scale industrially for the production of styrene monomer. Ethylbenzene may be produced by a number of different chemical processes but one process which has achieved a significant degree of commercial success is the vapor phase alkylation of benzene with ethylene in the presence of a solid, acidic ZSM-5 zeolite catalyst. In the commercial operation of this process, the polyalkylated benzenes, including both polymethylated and polyethylated benzenes are recycled to the alkylation reactor in which the reaction between the benzene and the ethylene takes place. By recycling the by-products to the alkylation reaction, increased conversion is obtained as the polyethylated benzenes (PEB) are converted to ethylbenzene (EB). In addition, the presence of the PEB during the alkylation reaction reduces formation of these species through equilibration of the components because at a given feed composition and under specific operating conditions, the PEB recycle will reach equilibrium at a certain level. This commercial process is known as the Mobil/Badger process and is described in more detail in an article by Francis G. Dwyer, entitled "Mobil/Badger Ethylbenzene Process-Chemistry and Catalytic Implications", appearing on pages 39-50 of a book entitled Catalysis of Organic Reactions, edited by William R. Moser, Marcel Dekker, Inc., 1981.
Ethylbenzene production processes are described in U.S. Pat. Nos. 3,751,504 (Keown), 4,547,605 (Kresge), and 4,016,218 (Haag); reference is made to these patents for a detailed description of such processes. The process described in U.S. Pat. No. 3,751,504 is of particular note since it includes a separate transalkylation step in the recycle loop which is effective for converting a significant proportion of the more highly alkylated products to the desired ethylbenzene product. Other processes for the production of ethylbenzene are disclosed in U.S. Pat. Nos. 4,169,111 (Wight) and 4,459,426 (Inwood), in both of which a preference for large-pore size zeolites such as zeolite Y is expressed, in distinction to the intermediate-pore size zeolites used in the processes described in the Keown, Kresge and Haag patents. U.S. Pat. No. 3,755,483 (Burress) describes a process for the production of ethylbenzene using zeolite ZSM-12 as the alkylation catalyst.
Ethylbenzene (EB) can be synthesized from benzene and ethylene (C.sub.2 =) over a variety of zeolitic catalysts in either the liquid phase or in the vapor phase. An advantage of a liquid phase process is its low operating temperature and the resulting low content of by-products. For example, U.S. Pat. No. 4,891,458 describes the liquid phase synthesis of ethylbenzene with zeolite Beta.
To minimize polyalkylation and other undesirable side reactions, production of such alkylaromatics as ethylbenzene and cumene typically operates with high ratios of aromatic (e.g., benzene) to alkylating agent (e.g., ethene) in the feed to the alkylation reactor. Zeolite-catalyzed processes generally operate at aromatic/olefin (A/O) molar feed ratios of three or above, while processes catalyzed by AlCl.sub.3 or supported phosphoric acid often operate at A/O's of three and below. However, in both cases polyalkylated aromatics (e.g., diethylbenzenes) are produced at levels that prohibit simply disposing of them as waste. These polyalkylated aromatics are instead reacted further with feed aromatic to form additional monoalkylate via transalkylation reactions. In the case of zeolite-catalyzed alkylaromatic processes, this transalkylation may take place in the alkylation reactor (as in the case of the Mobil/Badger process discussed above) or may take place in a separate transalkylation reactor, with the effluent from the transalkylation reactor being combined with the alkylation reactor effluent and sent to product recovery.